postgres materialized view support

Attempting to sync a database with a materialized view.

Expected result: rows in materialized view appear in index.
Actual result: document type does not appear in index.

Looks like [adaptor queries](https://github.com/appbaseio/abc/blob/2b659e2502a4c5788254924590a18bab38c5be47/adaptor/postgres/reader.go#L63) via
```
SELECT table_schema,table_name FROM information_schema.tables
```

Perhaps this is a viable alternative?
```
SELECT table_schema,table_name FROM information_schema.tables
UNION
SELECT schemaname, matviewname FROM pg_matviews
```

Version: 0.6.0
Variant: !oss
Go version: go1.10.1
OS: linux
Arch: amd64

##### Checks

- [x] I have mentioned my `abc` version in the issue, if applicable.
- [x] This issue is not a duplicate.
- [x] This issue has a description.

Start in adaptor/postgres/reader.go around the information_schema.tables query mentioned in the issue, then reproduce a sync against a PostgreSQL materialized view. Verify that the materialized view's rows and document type appear in the resulting index, alongside the existing table behavior.

Written by the indexing model from the issue text.